(a) Field of the Invention
The present invention relates to an apparatus for running a vehicle having an automatic transmission at a constant speed.
(b) Description of the Prior Art
A conventional apparatus for running a vehicle having an automatic transmission at a constant speed uses a diaphragm actuator or a servo motor to open or close a throttle valve to attain the constant speed of the vehicle.
FIG. 1 shows a block diagram of such a conventional apparatus for running the vehicle at the constant speed. In FIG. 1, numeral 10 denotes a reed switch for detecting a running velocity of a car and numeral 12 denotes a permanent magnet which is rotated by a speed meter cable. As the permanent magnet 12 is rotated, the reed switch 10 is turned on and off to produce a pulse signal having a frequency proportional to the car velocity. The pulse signal is converted to a D.C. voltage having a level proportional to the car velocity of a frequency-voltage conversion circuit (F/V converter) 14. The output voltage from the F/V converter 14 is supplied to a memory circuit 18 during an on-time of an analog switch 16 and a value of the output voltage immediately before the analog switch 16 is turned off is stored in the memory circuit 18 as a setting voltage representing a velocity to be maintained. The memory circuit 18 is usually constructed by a capacitor, and the setting voltage stored in the capacitor is compared with the output voltage of the F/V converter 14, that is, a car velocity voltage representing the car velocity in a compare circuit 20. The compare result is fed to an actuator 28 as a drive signal through an AND circuit 24 and a drive circuit 26. A control value 28a of the actuator 28 is a driven by the drive signal.
The drive signal for controlling the drive to the actuator 28 is a pulse signal having a duty factor which is variable with a difference between the setting voltage and the car velocity voltage. When the car velocity voltage is higher than the setting voltage, a duty cycle for the control valve 28a is reduced, and when the former is lower than the latter, the duty cycle is increased.
The AND circuit 24 is opened only when an output of a self-hold circuit 34 is logical "1" to supply the output of the compare circuit 20 to the drive circuit 26.
A setting switch 38 for setting the car velocity produces a logical "0" signal when it is actuated by a driver and produces a logical "1" signal when it is deactuated. The output of the setting switch 38 is supplied, through an inverter 40, to the analog switch 16 and the self-hold circuit 34. Accordingly, the analog switch 16 is turned on when the setting switch 38 is actuated and turned off when it is deactuated. The self-hold circuit 34 is also set when the setting switch 38 is actuated.
The self-hold circuit 34 may be a flip-flop which self-holds the setting operation by the setting switch 38 and supplies the output to the drive circuit 36 and enables the AND circuit 24. The self-hold circuit 34 is reset when one of a stop lamp switch 44 which is turned on when a brake pedal is stepped on, a parking brake switch 46 which is turned on when a parking brake is activated, or a neutral start switch 48 which is turned on when a shift lever of an automatic transmission is shifted to a neutral position. As a result, the drive circuit 36 is deactivated and the AND circuit 24 is disabled.
The actuator 28 has the control valve 28a and a release valve 28b. The control valve 28a is controlled by the output of the AND circuit 24 through the drive circuit 26. When the self-hold circuit 34 is set and the drive circuit 26 is activated, the control valve 28a cuts off an atmospheric pressure from a port 28c and introduces a suction pipe vaccum pressure from a port 28d into a chamber 28e. On the other hand, when the drive circuit 26 is deactivated, the control valve 28a cuts off the vacuum pressure from the port 28d and introduces the atmospheric pressure from the port 28c into the chamber 28e. A ratio of introduction of the atmospheric pressure from th port 28c and the suction pipe vacuum pressure from the port 28d is controlled by a duty factor of the output pulse from the compare circuit 20. The release valve 28b is controlled by the output of the self-hold circuit 34 through the drive circuit 36. When the self-hold circuit 34 is set and the drive circuit 36 is activated, the release valve 28b cuts off an atmospheric pressure from a port 28f, and when the self-hold circuit 34 is reset and the drive circuit 36 is deactivated, the release valve 28b introduces the atmospheric pressure into the chamber 28e. In this manner, the pressure in the chamber 28e is controlled so that a diaphragm 28g is moved. As a result, a rod 28h linked to an accelerator link, not shown, is axially moved to control an aperture of the throttle valve to maintain the car velocity at a constant speed.
The car equipped with this apparatus, however, cannot maintain the preset car velocity in some cases when the car goes up a steep slope. The constant speed maintaining apparatus having the diaphragm actuator shown in FIG. 1 cannot follow the present car velocity when the car goes up the steep slope and hence it cannot maintain the constant speed run. When the diaphragm actuator is used, the steeper is the slope the more does the load to the vehicle increase, and hence the rotation speed of the engine reduces. As a result, the engine suction pressure decreases and the throttle valve cannot be fully drawn.
When a servo motor is used, the throttle valve can be fully drawn but because of a small transmission ratio of the vehicle a drive torque cannot follow the gradient of the slop and a traction force is not sufficiently large. As a result, the actual vehicle speed slows down from the preset vehicle speed.